1. The Field of the Invention
The present invention relates to a heat exchanger for use in regulating the temperature of blood flowing in an extracorporeal circuit, such as through a bubble blood oxygenator.
2. The Prior Art
Many types of heart surgery require the use of an extracorporeal cardiopulmonary bypass circuit while the surgery is being performed. The role of such a circuit is essentially to take over the function of the heart and lungs while the surgical procedure is performed. This type of circuit is used to oxygenate and pump blood, just as the heart and lungs normally oxygenate and pump blood.
The basic components of such extracorporeal bypass circuits are, as would be expected, a pump which forces the blood through the circuit and back into the patient's body and an oxygenator which adds oxygen to the blood while providing for the removal of carbon dioxide. The most commonly used process for oxygenation of blood involves mixing small bubbles of oxygen rich gas with the blood in such a manner that the oxygen can be absorbed by the blood. Although many different types of "bubble oxygenators" have been developed, typically bubble oxygenators consist of an oxygenating section, a defoaming section and an arterial reservoir.
Oxygen and other gases are introduced into the oxygenating section through small tubes or a porous member. Each small tube or porous member creates small bubbles which are dispersed in the blood. As the gases and blood are mixed together, oxygen is absorbed by the blood and carbon dioxide is liberated. In most devices, the majority of the oxygenation takes place in this section. However, in some devices the blood has a short residence time in the oxygenating section; thus, oxygenation continues to occur as the blood passes through the subsequent defoaming section.
As oxygen is bubbled through the blood in a bubble oxygenator, a certain amount of foaming necessarily occurs. This foam, and any entrapped air bubbles, must be removed from the blood before it is reinjected into the patient; otherwise, the entrapped air bubbles can form an ambolus which can do sever damage to the patient. Defoaming is generally accomplished by passing the blood over a material having a large surface area which has been treated with a defoaming agent.
The blood next flows into the arterial reservoir. The arterial reservoir provides an area where the defoamed blood is collected before reinjection into the patient. The reservoir acts as a safety feature in helping avoid accidental pumping of air into the blood lines. Should the blood supply to the oxygenator be accidently stopped, the reservoir must contain sufficient blood to allow the perfusionist to stop the output from the oxygenator before air enters the arterial line.
During heart surgery while blood is flowing through the extracorporeal circuit, including the oxygenator, it becomes critical to have the capability of controlling the temperature of the blood. As would be expected, blood flowing outside the body for a significant period of time will tend to cool to room temperature. This cooling of the blood may or may not be desirable at any particular point in the surgery, however, it is clear that when the surgery is being completed it is desirable to warm the blood to normal body temperature in order to bring the patient back to normal temperature.
In addition, it is now common practice to use hypothermia during heart surgery. Reducing body temperature can significantly reduce the demand for oxygen by various vital organs. Specifically, the literature indicates that a patient's oxygen demand is decreased to about one-half at 30.degree. C., one-third at 25.degree. C., and one-fifth at 20.degree. C. Hypothermia is particularly useful in protecting organs such as the kidneys, heart, brain and liver which have a high demand for oxygen and require a high degree of perfusion. Mild (37.degree.-32.degree. C.), moderate (32.degree.-28.degree. C.), deep (28.degree.-18.degree. C.), and profound (18.degree.-0.degree. C.) hypothermia have all been employed. It has been found that moderate hypothermia is usually sufficient in a routine open-heart case. However, deep and profound hypothermia are sometimes advocated, particularly in cases of surgical repair of congenital defects in infants and small children.
One of the primary concerns in the use of hypothermia is the time which it takes to cool and reheat the blood. The time it takes to change the blood temperature is time added onto the length of the surgical procedure. If this time is minimized there is more time to perform the open-heart procedure and the total time taken to perform the operation is minimized. It is clear that reducing these time periods reduces the general trauma suffered by the patient. As a result, it is important that the heat exchanger be as efficient as possible in changing blood temperature.
It can be seen, therefore, that a means for controlling blood temperature in the extracorporeal circuit becomes critical, particularly when hypothermia is employed. Various types of heat exchanging devices have been used in order to attempt to regulate blood temperature during heart surgery.
Many early heat exchangers were placed in the extracorporeal circuit in addition to the oxygenator, rather then being incorporated within the oxygenator itself. In other words, the heat exchanger was a third major element in the circuit. The resulting circuit was comprised of a pumping mechanism, an oxygenator and a heat exchanger. The heat exchanger could take a variety of forms. Typical heat exchangers employed hollow coils or plates through which a heat exchange medium was circulated. The heat exchange medium was typically tap water, readily available in the operating room.
Subsequently, the heat exchanger and the oxygenator were attached within a single structure, typically made of molded plastic. The design of the oxygenator and the heat exchanger remained essentially the same. Blood would first flow through the oxygenator and, when completely oxygenated, would flow into the heat exchanger. However, linking the two elements within a single structure eliminated some problems previously encountered. Various segments of tubing could be eliminated and the oxygenator and heat exchanger were positioned so that the blood could readily travel through both without constant concern about positioning the various parts of the circuit.
Later devices incorporated the heat exchanger and the blood oxygenator into a unitary device. In these devices it was the general practice to have the blood mix with oxygen in the oxygenating section and then travel into a heat exchanger section. Following the heat exchanger section, the blood would flow through the defoaming section and in to the arterial reservoir for reinjection into the patient. The heat exchange function and the oxygenating function were thus combined into a single device. However, the heat exchanger, even though incorporated into the oxygenator, was still of the same general type used before.
Examples of this type of oxygenator-heat exchanger configuration include the Harvey and Shiley oxygenators. The Harvey oxygenator is a disposable, hard-shell, concentric, bubble oxygenator. See, Brumfield, "A Bubble Oxygenator and Heat Exchanger," U.S. Pat. No. 3,768,977 (Issued Oct. 23, 1973). The oxygenator is hung with a mounting bracket which holds the oxygenator at the top and bottom. Venous return and cardiotomy drainage, as well as gas, flows to the bottom of the oxygenator. The oxygen disperser is a sintered plate which produces bubbles of various sizes. The oxygenated blood travels upward through a series of parallel vertical tubes. An integral heat exchanger surrounds the tubes and is in contact with the blood path through the oxygenating as well as defoaming area, and in the arterial reservoir.
The Shiley oxygenator uses the same basic elements but employs a different heat exchanger. See, Lewin, "Blood Oxygenator with Integral Exchanger For Regulating the Temperature of Blood in an Extracorporeal Circuit," U.S. Pat. No. 4,065,264 (Issued Dec. 27, 1977). In the Shiley device the heat exchange medium travels through a coil. The blood reaches the coil, as in the Harvey device, after oxygenation but before defoaming. Positioned on the outside of the Shiley heat exchanger coil is a continuous hollow helical rib. The rib is employed in order to achieve more efficient heat exchange through increasing the residence time of the blood while traveling past the heat exchanger.
Other devices incorporate variations of the Shiley or Harvey concepts. For example, dual coils of the same general type use in the Shiley device have been used. In addition, several different configurations of the Harvey concept have been used including a series of parallel tubes running the length of the oxygenator. However, in order to obtain adequate heat exchange most prior devices employ convoluted and tortuous blood flow paths in order to increase heat exchange by attempting to increase blood residence time and decrease the boundary layer effect. However, none of the prior devices were able to provide adequate heat exchange without unduly increasing the surface area exposed to the blood, which in turn increases damage to the blood including hemolysis.
Pierre M. Galletti in the book Heart-Lung Bypass identified several criteria for an acceptable heat exchanger. The criteria are as follows:
(1) The material used in the heat exchanger should be non-toxic. It is clear that a material which leaches into the blood would be unacceptable. As a result many prior art devices have used stainless steel heat exchangers. While essentially non-toxic, stainless steel is not particularly blood compatible.
(2) The heat exchanger should be either easy to clean or disposable. It can be readily seen that devices using intricate flow paths including complex coil configurations could be extremely difficult to clean.
(3) There must be no internal leakage. If water or other heat exchange medium were allowed to leak into the blood, it is clear that the impact to the patient could be devastating.
(4) The heat exchanger should not significantly increase the blood flow resistance. It will be appreciated that the convoluted flow paths created in prior devices could not help but greatly increase flow resistance.
(5) The heat exchanger should cause a minimum of trauma to the blood. Again, the more involved the flow path and the tubing becomes, the more potential there is for trauma to the blood. In addition, exposing the blood to stainless steel is likely to cause more blood trauma than exposing the blood to more biocompatible surfaces such as various plastics.
(6) The heat exchanger must be efficient. Included within this criteria are the considerations of maximizing heat exchanged while minimizing surface area exposed and minimizing the volume of blood necessary to prime the oxygenator-heat exchanger.
(7) Finally, the heat exchanger must be reasonably inexpensive. No prior device fully accomplished the objects identified by Galletti.
Accordingly, what is needed in the art is a heat exchanger for use in connection with a bubble blood oxygenator which fulfills more closely the Galletti criteria. Particularly, what is needed is an efficient heat exchanger which does not employ a convoluted or tortuous flow path and which only exposes the blood to a biocompatible surface. It would be a further advancement in the art to provide a heat exchanger which is simple to construct and operate and which is easy and inexpensive to manufacture. Particularly, the heat exchanger should be easily incorporated into a standard bubble oxygenator. Such a device is disclosed and claimed herein.